Methods for reducing stress on composite structures

ABSTRACT

Methods for reducing stress on composite structures involving providing a primary composite structure having a circumference, providing at least one mounting flange operably connected to the primary composite structure about the circumference to form a joint, and providing a secondary structure operably connected to the primary composite structure at the mounting flange such that when stresses on the primary composite structure exceed a maximum capacity level delamination or separation of the mounting flange from the primary structure occurs at the joint, and the secondary structure remains operably connected to the mounting flange.

TECHNICAL FIELD

Embodiments described herein generally relate to methods for reducingstress on composite structures. More specifically, embodiments hereingenerally describe methods for reducing stress on composite structuresby providing a mounting flange on the composite structure thatdelaminates or separates, yet remains intact, after maximum stresscapacity has been exceeded.

BACKGROUND OF THE INVENTION

In gas turbine engines, such as aircraft engines, air is drawn into thefront of the engine, compressed by a shaft-mounted compressor, and mixedwith fuel in a combustor. The mixture is then burned and the hot exhaustgases are passed through a turbine mounted on the same shaft. The flowof combustion gas expands through the turbine which in turn spins theshaft and provides power to the compressor. The hot exhaust gases arefurther expanded through nozzles at the back of the engine, generatingpowerful thrust, which drives the aircraft forward.

Because engines operate in a variety of conditions, foreign objects mayundesirably enter the engine. More specifically, foreign objects, suchas large birds, hailstones, sand and rain may be entrained in the inletof the engine. As a result, these foreign objects may impact a fan bladeand cause a portion of the impacted blade to be torn loose from therotor, which is commonly known as fan blade out. The loose fan blade maythen impact the interior of the fan casing causing a portion of thecasing to bulge or deflect. This deformation of the casing may result inincreased stresses along the entire circumference of the engine casing.

In recent years composite materials have become increasingly popular foruse in a variety of aerospace applications because of their durabilityand relative lightweight. Although composite materials can providesuperior strength and weight properties, and can lessen the extent ofdamage to the fan casing during impacts such as blade outs, designingflanges on structures fabricated from composite materials still remainsa challenge.

Laminated composite structures generally have superior strength in-planedue to the presence of continuous reinforcing fibers. However, issuesmay arise when attaching a secondary structure to an interposing flangelocated about the body of the composite structure, as opposed to aboutan end of the composite structure. Such issues are due to a general lackof continuous fibers at the points of attachment, or joints, between theflange and primary composite structure. This, in addition to significantout-of-plane loads caused by the weight of the secondary structure, mayresult in a weak attachment joint that is susceptible to damage fromincreased stresses, such as those resulting from a fan blade out orthose inherently present due to the weight of the secondary structure.

To address such weaknesses at the point of attachment, it may bedesirable to provide supplementary reinforcement to the joints of themounting flange, such as additional fibers or metal brackets. However,with the addition of these reinforcements, the weight-saving benefitsprovided by using composite structures can be significantly reduced.Moreover, even with additional reinforcements, the mounting flange maystill not be strong enough to adequately support the weight of theattached secondary structure, with or without the additional stressescaused by a blade out. Ultimately, continuous stresses on the alreadyweakened flange may result in catastrophic failure to one or more of theprimary composite structure, the attached secondary structure, theengine or the aircraft.

Accordingly, there remains a need for methods for reducing stress oncomposite structures having mounting flanges that provide the desiredattachment without the previously described failure issues.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments described herein generally relate to methods for reducingstress on composite structures comprising providing a primary compositestructure having a circumference, providing at least one mounting flangeoperably connected to the primary composite structure about thecircumference to form a joint, and providing a secondary structureoperably connected to the primary composite structure at the mountingflange wherein when stresses on the primary composite structure exceed amaximum capacity level delamination of the mounting flange from theprimary structure occurs at the joint and the secondary structureremains operably connected to the mounting flange.

Embodiments herein also generally relate to methods for reducing stresson composite structures comprising providing a primary compositestructure having a circumference, providing at least one mounting flangeoperably connected to the primary composite structure about thecircumference to form a joint, and providing a secondary structureoperably connected to the primary composite structure at the mountingflange wherein when stresses on the primary composite structure exceed amaximum capacity level separation of the mounting flange from theprimary structure occurs at the joint and the secondary structureremains operably connected to the mounting flange.

These and other features, aspects and advantages will become evident tothose skilled in the art from the following disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the invention, it is believed that theembodiments set forth herein will be better understood from thefollowing description in conjunction with the accompanying figures, inwhich like reference numerals identify like elements.

FIG. 1 is a schematic cross-sectional view of one embodiment of a gasturbine engine;

FIG. 2 is a schematic view of one embodiment of a fan casing having amounting flange;

FIG. 3 is a schematic perspective view of one embodiment of a compositestructure forming tool;

FIG. 4 is a schematic cross-sectional view of one embodiment of amounting flange operably connected to a fan casing;

FIG. 5 is a schematic view of one embodiment of a fan casing havingmounting flanges and attached secondary structure;

FIG. 6 is a schematic representation of one embodiment of a process forfabricating a mounting flange;

FIG. 7 is a schematic cross-sectional view of one embodiment of a flangeshoe;

FIG. 8 is a schematic cross-sectional view of one embodiment of anextended flange shoe;

FIG. 9 is a schematic cross-sectional view of one embodiment of toolingused during final cure of a composite fan casing having two mountingflanges; and

FIG. 10 is a portion of a schematic cross-sectional view of oneembodiment of tooling used during final cure of a composite fan casinghaving two adjacent mounting flanges.

DETAILED DESCRIPTION OF THE INVENTION

Integral Composite Mounting Flanges

Embodiments described herein generally relate to methods for reducingstress on a composite structure that can reduce the occurrence of severepart damage to both a primary composite structure and an attachedsecondary structure, while concurrently helping to eliminatecatastrophic part failure. While embodiments herein may generally focuson integral mounting flanges on composite fan casings of gas turbineengines, it will be understood by those skilled in the art that thedescription should not be limited to such. Indeed, as the followingdescription explains, the integral mounting flange described herein maybe utilized on any generally cylindrically-shaped composite structure.

Turning to the figures, FIG. 1 is a schematic representation of oneembodiment of a gas turbine engine 10 that generally includes a fanassembly 12 and a core engine 14. Fan assembly 12 may include a fancasing 16 and an array of fan blades 18 extending radially outwardlyfrom a rotor disc 20. Core engine 14 may include a high-pressurecompressor 22, a combustor 24, a high-pressure turbine 26 and alow-pressure turbine 28. Engine 10 has an intake side 30 and an exhaustside 32. Fan assembly 12 and low-pressure turbine 28 may be coupled by afirst rotor shaft 34 while high-pressure compressor 22 and high-pressureturbine 26 may be coupled by a second rotor shaft 36.

FIG. 2. illustrates one embodiment of an acceptable primary compositestructure 38. As used herein, “composite structure (preform)” refers toany component, or preform thereof, fabricated from composite materials.Composite structure 38 may comprise a generally cylindrical member, suchas fan casing 16. Henceforth, the cylindrical member will be referred toas fan casing 16, though it should not be limited to such. Fan casing 16may be generally cylindrical in shape and may be fabricated from anyacceptable material. In one embodiment, however, fan casing 16 may befabricated from a composite material, such as, but not limited to, glassfibers, graphite fibers, carbon fibers, ceramic fibers, aromaticpolyamide fibers such as poly(p-phenylene terephthalamide) fibers (i.e.KEVLAR®), and combinations thereof. In one embodiment, the compositematerial may comprise carbon fibers. Additionally, fan casing 16 may befabricated using any acceptable fabrication method known to thoseskilled in the art. See, for example, U.S. Patent Application No.2006/0201135 to Xie et al.

Fan casing 16 may generally comprise a body 40 having a forward end 42and an aft end 44. As used herein, “fan casing” is used to refer to bothpre- and post-cure composite fan casings. Those skilled in the art willunderstand which stage is being referenced from the present description.Fan casing 16 may also comprise at least one integral composite mountingflange 46. As used herein, “mounting flange” refers to any flangeinterposed circumferentially about body 40 of fan casing 16, or otherprimary composite structure, that may be used to operably connect asecondary structure to the primary structure, as described herein below.By “interposed” it is meant that mounting flange 46 may be locatedcircumferentially about body 40 of fan casing 16, as opposed to abouteither of forward end 42 or aft end 44.

Fan casing 16 may also be fabricated using any tool known to thoseskilled in the art. See, for example, U.S. Patent Application No.2006/0134251 to Blanton et al. In one embodiment, as shown in FIG. 3,composite structure forming tool 37 may have a circumference, agenerally cylindrically shaped core 33, and comprise a first endplate 72and a second endplate 84 that may be removeably attached to core 33 oftool 37.

Turning to FIG. 4, integral composite mounting flange 46 may generallyinclude at least one core fiber 52, though in one embodiment mountingflange 46 may comprise a plurality of core fibers 52. Core fibers 52 maybe circumferentially oriented about fan casing 16. By “circumferentiallyoriented” it is meant that core fibers 52, whether fiber tows, textilepreforms or a combination thereof, generally circumscribe fan casing 16and are continuous in the circumferential direction. Mounting flange 46may also generally comprise at least one layer of multidirectionalattachment fibers 54 that may operably connect core fibers 52 to fancasing 16 as described herein below. As used herein, “multidirectional”refers to textile preforms comprising the attachment fibers that havefiber tows oriented in more than one direction.

As will be understood by those skilled in the art, core fibers 52 may befabricated in different ways. In one embodiment, core fibers 52 may befabricated from a plurality of continuous, unidirectional fiber towsbundled and bonded together. In another embodiment, core fibers 52 maycomprise textile preforms, such as a flattened biaxial braid sleeve,having a majority of fiber tows that are continuous in thecircumferential direction, and the remaining fibers either continuous ornon-continuous in the non-circumferential direction. It is this generalcircumferential orientation of core fibers 52 that can provide addedstrength to the flange in the circumferential direction as explainedherein below. Regardless of the particular assembly utilized, corefibers 52 may comprise a first core side 56 and a second core side 58.

Fiber tows of core fibers 52 may be comprised of any suitablereinforcing fiber known to those skilled in the art, including, but notlimited to, glass fibers, graphite fibers, carbon fibers, ceramicfibers, aromatic polyamide fibers such as poly(p-phenyleneterephthalamide) fibers (i.e. KEVLAR®), and combinations thereof.Additionally, while any number of fiber tows may be used to constructcore fibers 52, in one embodiment there may be from about 100 to about5000 fiber tows used to construct core fibers 52. Moreover, each fibertow may comprise from about 3000 to about 24,000 fiber filaments. Ingeneral, when assembled, core fibers 52 may constitute about half of theoverall thickness T of mounting flange 46. While the thickness ofmounting flange 46 may vary according to application, in one embodiment,mounting flange 46 may have a thickness of from about 0.5 inches (1.27cm) to about 1 inch (2.54 cm).

As explained previously, in addition to circumferential core fibers 52,each mounting flange 46 may also include at least one layer ofattachment fibers 54 operably connecting each of first core side 56 andsecond core side 58 of core fibers 52 to fan casing 16. Unlike corefibers 52, attachment fibers 54 may be constructed of multidirectionaltextile preforms, such as weaves or braids, that need not have amajority of fiber tows oriented circumferentially. In this way,attachment fibers 54 can display a generally uniform strengthdistribution throughout. As with the core fibers, each fiber tow ofattachment fibers 54 may comprise from about 3000 to about 24,000 fiberfilaments. Generally, when assembled, attachment fibers 54 mayconstitute the remaining half of the overall thickness of flange 46.

As illustrated in FIG. 5, mounting flange 46, once cured, may be used tooperably connect at least one secondary structure 48 to fan casing 16and thus, flange 46 may be located in a variety of locations along thelength of body 40 of fan casing 16. In some instances, it may bedesirable to include more than one mounting flange 46. As shown in FIG.5, in one embodiment, secondary structure 48 may be, for example, anaccessory gear box 50 that can be mounted to fan casing 16 using themounting flanges 46 and any attachment method known to those skilled inthe art, such as bolts. Other possible secondary structures may include,but are not limited to, an oil tank, oil and fuel monitoring modules,other engine externals and combinations thereof. It will be understoodthat “engine externals” refers to any accessory, module or componentthat may be connected to the outside of the engine. Such secondarystructures may be constructed of any acceptable material known to thoseskilled in art such as, for example, aluminum, and as describedpreviously may weigh significantly more than the corresponding fancasing to which they are attached. For example, in one embodiment, fancasing 16 may weight about 200 pounds while accessory gear box 50 mayweigh about 300 pounds.

Embodiments of the mounting flange described herein can provide severalbenefits over existing attachment mechanisms. In particular, theintegral mounting flange can reduce the occurrence of severe part damageto both the primary composite structure, as well as the attachedsecondary structure, while concurrently helping to eliminatecatastrophic part failure. Without intending to be limited by theory, itis believed that, in general, fiber-reinforced composite structures,such as the mounting flanges herein, can have relatively weak interfacesbetween fiber layers and, therefore, have relatively weakthrough-thickness strength compared to their in-plane strength. Ifstresses on the composite structure exceed a defined maximum capacitylevel, these fiber layers can have a tendency to delaminate, orseparate, prior to actual fiber breakage occurring. This delamination orseparation can reduce the load and stress on the attachment joint wherethe mounting flange connects to the primary structure. As will beunderstood by those skilled in the art the maximum stress capacity levelof the primary composite structure can vary depending on such factors asmaterials of fabrication, method of fabrication and the like.

Embodiments set forth herein are designed take advantage of thepreviously described phenomenon. More specifically, the integralmounting flange may be fabricated to permit delamination, or evenseparation, of the flange from the primary composite structure at thejoint under excessive stresses, such as those caused by a fan blade outor by the weight of an attached secondary structure. However, becausethe core fibers of the flange can be constructed from continuous,circumferentially oriented fibers, even after delaminating or separatingthe flange can remain a movable yet intact ring about the primarystructure. Thus, even if the integral mounting flange delaminates orseparates from the primary composite structure, it generally remains inplace with all secondary structures attached. This can allow stresses onboth the primary composite structure and the mounting flange to bereduced while maintaining the attached secondary structure in the samegeneral placement as originally intended. Because of this, thedelamination or separation can reduce damage to both the primary andsecondary structures, as well as help to prevent catastrophic partfailure.

Methods of Fabricating Integral Composite Mounting Flange

Fabricating a mounting flange as set forth herein may generally compriseapplying core fibers about the primary composite structure, followed byapplying attachment fibers to operably connect the core fibers to thefan casing, or other primary composite structure. More specifically, asshown in FIG. 6 step 100, the fabrication of a mounting flange may beginwith providing a primary composite structure having a circumference,such as fan casing 16. In one embodiment, the primary compositestructure may be complete except for final cure. In step 102, acorrespondingly shaped guide 60 may then be placed about body 40 of fancasing 16 in each location where a mounting flange is desired. Guide 60may be removably held in place by shrink tape, for example. In oneembodiment, guide 60 may be comprised of discrete arcuate members, eachspanning about 180 degrees of body 40 of fan casing 16. The arcs ofguide 60 may be releaseably connected together for easy placement andadjustment about fan casing 16. It will be understood, however, thatguide 60 may be comprised of any number of pieces and have any shapethat corresponds to the shape of the primary composite structure. Guide60 can serve as a support for the later application of both the corefibers and the attachment fibers, as explained herein below. Aspreviously mentioned, guide 60 may be circumferential and have anL-shaped cross-section as shown, and may be constructed from any rigid,lightweight material such as, for example, aluminum or composite.

In step 104, once all guides 60 have been placed in the desiredlocations about body 40 of fan casing 16, the application of core fibers52 may be initiated. As previously discussed, core fibers 52 maycomprise either unidirectional, circumferentially oriented fiber towsbundled and bonded together or textile preforms, such as a flattenedbiaxial braid sleeve, having a majority of continuous, circumferentiallyoriented fiber tows.

If unidirectional fiber tows are used to construct core fibers 52, thetows may comprise fiber filaments that can be wound about the fan casing16. In general, a single tackified fiber tow can be precisely placed inthe desired position about the fan casing and this process can berepeated until core fibers 52 have the desired size and shape. Adebulking step may then be carried out to consolidate core fibers 52, asdescribed herein below. Alternately, if textile preforms are used toconstruct core fibers 52, the textile layers can be layed-up andtackified on a flat, non-porous surface, such as a table or a tool. Morespecifically, the tackified textile layers can be stacked to form thecore fibers 52 desired thickness and height, while still being longenough to circumscribe the fan casing. After debulking, as set forthbelow, the consolidated textile layers remain flexible enough to allowthe layers to be manually or mechanically shaped into the proper radiusto fit the fan casing, or other primary composite structure. Regardlessof which type of fibers are used, finished core fibers 52 may have firstcore side 56 and second core side 58.

Having positioned core fibers 52 in the desired location about fancasing 16, attachment fibers 54 may be applied to each of first coreside 56 and second core side 58 of core fibers 52, as well as to fancasing 16 to operably connect core fibers 52 to fan casing 16. In step106, guide 60 can be left in place while attachment fibers 54 areapplied to, for example, first core side 56 of core fibers 52. Aspreviously described, attachment fibers 54 may comprise multidirectionaltextile preform layers, such as weaves or braids. Layers of attachmentfibers 54 may be wrapped against both first core side 56 of core fibers52 and fan casing 16 until the desired thickness is obtained. Morespecifically, a liquid resin, such as an epoxy, may be applied to corefibers 52 and fan casing 16 to provide a tacky layer to which attachmentfibers 54 may be applied. Next, a layer of attachment fibers 54 may beapplied over the liquid resin. This process can be repeated until thedesired thickness of attachment fibers 54 is achieved. Though attachmentfibers 54 may have any thickness, in one embodiment, the thickness ofattachment fibers may be from about 0.125 inches (about 0.3 cm) to about0.25 inches (about 0.6 cm).

Once attachment fibers 54 have been applied to first core side 56 ofcore fibers 52 a debulk may again be performed to consolidate theconstruction thus far. In particular, reinforcing fibers, such as corefibers 52 and attachment fibers 54, may inherently have a substantialamount of bulk. In order to help prevent wrinkles and/or voids duringthe final cure of the composite, and to utilize near net shape toolingduring the final cure, the fibers of the composite can be consolidated,or compressed, into a dimension that is closer to the desired finalcured thickness. This consolidation occurs during debulk.

Debulk can be carried out using any common method known to those skilledin the art, such as, for example, by applying pressure to the compositefibers with either a vacuum bag, shrink tape, or other mechanical means.Resin applied to the fibers before debulk can help “tack,” or lock, thefibers in place once the pressure is applied. If the tackified fiberscannot be consolidated as desired at room temperature, then heat may beapplied to lower the viscosity of the resin. The resin may then betterinfiltrate the composite fibers and allowing the consolidation to becarried out to the desired degree. In one embodiment, the guide may beleft in place during the debulk process to provide support duringfabrication.

After debulk, guide 60 may be repositioned adjacent to the completedside of the flange for the application of attachment fibers 54 to theopposing side of the flange as shown in step 108. The previouslydescribed application and debulk of attachment fibers 54 may then berepeated on, for example, second core side 58 of core fibers 52, toobtain an integral composite mounting flange perform 61 in step 110.

Optionally, in one embodiment shown in step 112, additional individualfiber tows 62 may be applied to attachment fibers 54 of mounting flangepreform 61 prior to final cure to provide additional hoop strength. Suchfiber tows will not affect the final cure of the composite structure.However, to avoid limiting the weight-saving benefits provided by usingcomposite materials, it may be desirable to minimize the use ofadditional individual fiber tows 62.

Once core fibers 52, attachment fibers 54, and optionally individualfiber tows 62, have been layed-up and debulked, each guide 60 can beremoved and the final cure tooling can be placed about fan casing 16,including any flange performs, to serve as a mold during the curingprocess. As will be understood by those skilled in the art, the finalcure tooling and process may vary according to such factors as resinused, part geometry, and equipment capability. However, in oneembodiment, the tooling may comprise near net shape tooling, which notonly helps prevent waste of raw material and machining time, but alsoeliminates having to machine into the attachment fibers, which couldresult in breaking the fibers and introducing weak points in the flange.

In general, the final cure tooling 64 may comprise various combinationsof flange shoes and extended flange shoes. Flange shoes 66 may compriseany number of pieces that when coupled together may be positionedcircumferentially about fan casing 16, and optionally mounting flangeperforms 61, and may comprise a substantially L-shaped cross-section, asshown in FIG. 7. Extended flange shoes 68, shown in FIG. 8, may have afirst side 69 and a second side 71 and may also comprise any number ofpieces that when coupled together may be positioned circumferentiallyabout fan casing 16, and optionally mounting flange performs 61.Extended flange shoes 68 may comprise a substantially U-shapedcross-section, as shown in FIG. 8. Both flange shoes 66 and extendedflange shoes 68 may be constructed of any material having a greaterthermal coefficient of expansion than the fan casing preform. In oneembodiment, flange shoes 66 and extended flange shoes 68 may beconstructed from metals, alloys or combinations thereof, such asaluminum or steel. Additionally, as explained herein below, either orboth of flange shoes 66 and extended flange shoes 68 may comprise aflange cavity to accommodate an end flange preform or a mounting flangepreform.

More particularly, as shown in FIG. 9, a first extended flange shoe 70may be placed about fan casing 16 such that first side 69 of firstextended flange shoe 70 is adjacent to a first endplate 72 of thecomposite structure-forming tool 74 upon which fan casing 16 isfabricated. First extended flange shoe 70 may be removeably coupled tothe first endplate 72 using any attachment method known to those skilledin the art, such as, for example, bolts. Once positioned, first extendedflange shoe 70 may overlay any first end flange preform 76 present, andcontinue along body 40 of fan casing 16 to a first mounting flangepreform 78, as shown in FIG. 9. A first flange shoe 80 may then bepositioned about fan casing 16, adjacent to second side 71 of firstextended flange shoe 70 and the two may be removeably coupled togetherabout first mounting flange preform 78. In this way, first extendedflange shoe 70 can serve as an endplate to first flange shoe 80 andprovide the support necessary to help ensure first flange shoe 80remains in position such that first mounting flange preform 78 retainsits desired shape and orientation about fan casing 16 during final cure.

As also shown in FIG. 9, a second extended flange shoe 82 may be placedabout fan casing 16 such that a first side 69 of second extended flangeshoe 82 is adjacent to a second endplate 84 of compositestructure-forming tool 74. Second extended flange shoe 82 may beremoveably coupled to the second endplate 84 in the same manner providedabove for first extended flange shoe 70. Again, second extended flangeshoe 82 may overlay any second end flange preform 86 present, andcontinue along body 40 of fan casing 16 to a second mounting flangepreform 88, as shown in FIG. 9. A second flange shoe 90 may then bepositioned about fan casing 16 adjacent to second side 71 of secondextended flange shoe 82 and the two may be removeably coupled togetherabout second mounting flange preform 88. As before, second extendedflange shoe 82 can provide support to second flange shoe 90 and helpensure second flange shoe 90 remains in position such that secondmounting flange preform 88 retains its desired shape and orientationabout fan casing 16 during final cure.

For each coupling of an extended flange shoe and a flange shoe, theremay also be a flange-shaped cavity formed to accommodate any mountingflange preform. It will be understood by those skilled in the art thatcavity may be formed in a flange shoe, an extended flange shoe, or acombination thereof. For example, in FIG. 9, first extended flange shoe70 contains a cavity 92 to accommodate first mounting flange preform 78while second flange shoe 90 contains a cavity 92 to accommodate secondmounting flange preform 88. Additional cavities 92 may be included toaccount for end flanges if present.

As shown in FIG. 10, if adjacent mounting flange performs are present, afirst extended flange shoe 70, having a first side 69 and second side71, may be removeably coupled to a second extended flange shoe 82,having a first side 69 and second side 71, to form a cavity 92therebetween about a first mounting flange preform 78. A first flangeshoe 80 may then be removeably coupled to second side 71 of secondextended flange shoe 82 in the manner described above to form anothercavity 92 about a second mounting flange preform 88. Indeed, any numberof extended flange shoes may be coupled together in this manner toaccommodate a fan casing having multiple adjacent mounting flangeperforms to help ensure the flange performs have the support needed toremain properly positioned and proportioned.

Once all flange shoes and extended flange shoes have been coupledtogether about the fan casing and the mounting flange performs, thefinal cure of the fan casing may commence. Those skilled in the art willunderstand how to determine the proper final cure parameters based onsuch factors as part size and resin utilized. At the end of the finalcure, the tooling may be removed and an article including a compositestructure having at least one mounting flange is obtained and anydesired secondary structure may then be attached thereto.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral language of the claims.

What is claimed is:
 1. A method for reducing stress on compositestructures comprising: providing a primary composite structure having acircumference and oppositely-disposed axial ends; providing at least oneinterposed mounting flange operably connected to the primary compositestructure about the circumference thereof and between the axial ends ofthe primary composite structure to form a weakened joint therebetweenthat permits the mounting flange to delaminate or separate from theprimary composite structure at the joint and yet remain intact aroundthe circumference of the primary composite structure and retainedthereon; wherein the step of providing the at least one interposedmounting flange comprises: forming the mounting flange to comprise aplurality of circumferentially oriented core fibers that circumscribethe primary composite structure, the core fibers having a first coreside and a second core side; and operably connecting each of the firstand second core sides of the core fibers to the primary compositestructure and about the circumference thereof with at least one layer ofattachment fibers; and operably connecting a secondary structure to themounting flange in predetermined relationship to the primary compositestructure at the mounting flange; wherein when stresses on the primarycomposite structure exceed a maximum capacity level, delamination orseparation of the mounting flange from the primary structure occurs atthe weakened joint and the secondary structure remains operablyconnected to the mounting flange.
 2. The method of claim 1, wherein theprimary composite structure comprises a material selected from the groupconsisting of glass fibers, graphite fibers, carbon fibers, ceramicfibers, aromatic polyamide fibers and combinations thereof.
 3. Themethod of claim 1, wherein the mounting flange comprises a materialselected from the group consisting of glass fibers, graphite fibers,carbon fibers, ceramic fibers, aromatic polyamide fibers andcombinations thereof.
 4. The method of claim 1, wherein the plurality ofcore fibers comprise unidirectional fiber tows.
 5. The method of claim1, wherein the plurality of core fibers comprise textile preforms havinga majority of fiber tows that are continuous in a circumferentialdirection with respect to the circumference of the primary compositestructure.
 6. The method of claim 1, wherein after delaminating orseparating, the mounting flange having the connected secondary structureremains an intact ring about the primary composite structure.
 7. Themethod of claim 1, wherein the primary composite structure comprises aturbine engine fan casing.
 8. The method of claim 1, wherein thesecondary structure is at least one engine external structure.